Method of forming a capacitance type gaseous sensing device and apparatus thereof

ABSTRACT

The capacitance type gaseous sensing device (10) includes a first electrode layer (12) formed on a semiconductor substrate layer (14). A seed layer (16) is formed on the first electrode layer (12). A reorganized layer (18) is formed on the first electrode layer (12) through interaction with the seed layer (16) to form a porous sensing layer. A second electrode layer (20) is formed on the reorganized layer (18). The reorganized layer (18) absorbs gaseous elements that change the dielectric constant of the capacitance type sensor device (10). A change in the dielectric constant causes a change in the capacitance of the reorganized layer (18) as measured across the first electrode layer (12) and the second electrode layer (20).

This is a division of application Ser. No. 08/329,003, filed on Oct. 25,1994.

TECHNICAL FIELD OF THE INVENTION

The present invention relates in general to devices for sensing gaseousemissions and more particularly to a method of forming a capacitancetype gaseous sensing device and apparatus thereof.

BACKGROUND OF THE INVENTION

Increasing concern over the daily emission of both toxic and greenhousegases has prompted the need for more sensitive and selective gassensors. Legislation restricting the current and future emission ofthese gases from sources such as internal combustion engines indirectlyrequires the improvement of sensors to detect gases such as carbondioxide, carbon monoxide, and nitrous oxide. Modern sensors rely on theinteractions between sensor and analyte to produce a detectable changein either the electrical, optical, mechanical, electrochemical, orthermal properties of the sensor. This interaction can take the form ofphysisorption, chemisorption, or catalysis.

A sensor that relies on physisorption has the advantage of being easilyrefreshed. However, physisorption involves weak attractive forces whichpresent difficulties in establishing analyte detection. Chemisorptionsensors rely on the chemical bonding of the analyte molecules to theprobe surface to produce a detectable change in the electricalconductivity mass, capacitance, or optical properties of that surface.Difficulties arise when the sensor is in continual use as it isnecessary to expose a fresh probe surface to the incoming analyte due tothe chemical bonding of the analyte molecules. Catalytic sensorstransform the analyte to a different compound, yielding a detectablechange of heat. A major disadvantage of catalytic sensors lie in theirinability to detect compounds that are chemically stable.

Low density molecular sieves have been shown to be useful as selectivechemical sensors. Zeolites are one of the several groups that comprisethe structural family of microporous metal oxides known as molecularsieves. Zeolites are crystalline aluminum silicates that have threedimensional networks of connected channels or cages. A molecular sievebased sensor might be used to selectively detect those molecules whichcan readily absorb into the zeolite cages.

Zeolite molecular sieves have been employed as the active component insurface acoustic wave devices and piezo electric quartz crystalmicrogravometric type sensors. These approaches involve either a thinfilm configuration as a ceramic composite or the zeolites are tetheredto an electrode surface via organic linkages. The sensing ability ofthese devices depends on changes in either mass or electrical propertiesproduced by the absorption of the analyte. The sensing characteristicsare limited in that a detectable mass change is required and thecircuitry needed to detect a change in mass is relatively complicated.Further, these approaches cannot discriminate between analyte moleculesof similar size and shape. Another problem is that it is very difficultto deposit uniform thin films of low density molecular sieve material inorder to fabricate a reliable gaseous sensor. Therefore, it is desirableto have a gaseous sensor that has a uniform film layer and candiscriminate between different types of absorbed analytes.

SUMMARY OF THE INVENTION

From the foregoing it may be appreciated that a need has arisen for agaseous sensor with a thin uniform film of a low density molecular sievematerial. A need has also arisen for a capacitance type sensor that candiscriminate between different absorbed analytes.

In accordance with the present invention, there is provided a method offorming a capacitance type gaseous sensing device and apparatus thereofare provided which substantially eliminate and reduce disadvantages andproblems associated with other physisorption, chemisorption, orcatalysis type sensors.

According to an embodiment of the present invention, there is provided amethod of forming a capacitance type gaseous sensing device thatincludes forming a first electrode layer onto a semiconductor substratelayer. A reorganized layer of a molecular sieve material is uniformlyformed onto the electrode layer while maintaining surface crystallinityof the reorganized layer. A second electrode layer is formed on selectedportions of the reorganized layer in order to establish the capacitancetype sensor. The uniformity of the reorganized layer may be achievedthrough a prior formation of a seed layer on the first electrode layer.

The present invention provides various technical advantages overphysisorption, chemisorption, or catalysis type sensors. For example,one technical advantage is in having a reliable capacitance type sensordevice. Another technical advantage is in having a gaseous sensor with auniform layer of a molecular sieve material. Yet another technicaladvantage is in using a seed layer to establish uniformity of thereorganized layer and provide a very thin layer for increasedcapacitance while allowing for selective absorption. Other technicaladvantages are readily apparent to one skilled in the art from thefollowing figures, description, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention and theadvantages thereof, reference is now made to the following descriptiontaken in conjunction with the accompanying drawings, wherein likereference numerals represent like parts, in which:

FIGS. 1A-B illustrate cross-sectional drawings depicting the processsteps in forming a capacitance type gaseous sensing device;

FIG. 2 illustrates a pulse laser ablation process used duringfabrication of the capacitance type gaseous sensing device;

FIG. 3 illustrates a hydrothermal process used in fabricating thecapacitance type gaseous sensing device; and

FIG. 4 illustrates a plot depicting capacitance-levels for differentgaseous elements absorbed by the capacitance type gaseous sensingdevice.

DETAILED DESCRIPTION OF THE INVENTION

FIGS. 1A-B show the process steps in forming a capacitance type gaseoussensing device 10. Capacitance type gaseous sensing device 10 is made byforming a first electrode layer 12 onto a semiconductor substrate 14. Aseed layer 16 is formed over portions of first electrode layer 12.Referring to FIG. 1B, a reorganized layer 18 is formed on seed layer 16and first electrode layer 12 to establish a pore sensing layer forcapacitance type gaseous sensing device 10. A second electrode layer 20is formed onto selected portions of reorganized layer 18.

In operation, capacitance type gaseous sensing device 10 absorbs gaseousemissions into the porous sensing layer formed by reorganized layer 18and seed layer 16. The absorption of gases within the porous sensinglayer causes a change in the dielectric constant of the porous sensinglayer. This change in the dielectric constant is measured as a change incapacitance across first electrode layer 12 and second electrode layer20. Different gases absorbed by the porous sensing layer cause differentchanges in capacitance for capacitance type gaseous sensing device 10.Only certain gases can be absorbed by certain molecular sieve materials,therefore the material used in the porous sensing layer can be selectedfor specific gaseous detection.

In order to register large capacitance values, the porous sensing layerof capacitance type gaseous sensing device 10 is of a thin dielectricfilm in order to maintain a small distance between first electrode layer12 and second electrode layer 20. Further, the very thin films of aneutral lattice molecular sieve form the material for the porous sensinglayer in order to achieve a sensitive and selective capacitance typechemical sensor. Molecular sieve materials that provide good sensingcharacteristics and used in forming the porous sensing layer includealuminum phosphates and metal aluminum phosphates. An aluminum phosphatebased molecular sieve material works well due to its insulatingproperties.

First electrode layer 12 and second electrode layer 20 preferably aremade of oxidation resistant or inert metal materials that stand up tothe formation of the porous sensing layer. Materials that satisfy thecharacteristics for first electrode layer 12 and second electrode layer20 include titanium nitride, gold, palladium, and platinum.

FIG. 2 shows a simplified block diagram depicting how seed layer 16 isformed onto first electrode layer 12. Seed layer 16 may be formed ontofirst electrode layer 12 by a pulse laser ablation process. The pulselaser ablation process may occur by directing a laser beam 22 onto atarget 24 made of the molecular sieve material. Target 24 is placed on amounting block 26 and laser beam 22 is directed across target 24 by arastering mirror 28. Rastering mirror 28 receives laser beam 22 from anexcimer laser 30 through a beam reducer 32. Rastering mirror 28 allowslaser beam 22 to be moved across target 24 instead of ablating materialfrom a single spot of target 24. Laser beam 22 is further reduced by alens 34 before entering a vacuum chamber 36 through a quartz window 38.A plume of material 40 is emitted from target 24 as a result of ablationcaused by laser beam 22. Plume 40 comes in contact with first electrodelayer 12, causing seed layer 16 to form thereon. Seed layer 16 is formedof molecular sieve fragments that form on portions of first electrodelayer 12. Semiconductor substrate 14, with first electrode layer 12, ismounted on a platform 42 that includes a lamp 44 and a thermocouple 46used to heat and set the temperature of capacitance type gaseous sensingdevice 10 during the ablation process. Vacuum chamber 36 may becontrolled through a gas manifold, the chamber pressure ambient beingmonitored with a pressure gauge 43.

FIG. 3 shows a hydrothermal process used in placing reorganized layer 18onto seed layer 16 and first electrode layer 12. After pulse laserablation, semiconductor substrate 14 is placed into a chamber 50 havingan aqueous solution 52 that includes the precursor chemicals required tomake the molecular sieve material. Chamber 50 may be pressurized throughpressure regulator 54 and heated by heating element 56 to allow forhydrothermal growth of gel mixture 52 onto seed layer 16 and firstelectrode layer 12 of capacitance type gaseous sensing device 10.Capacitance type gaseous sensing device 10 is preferably placed suchthat seed layer 16 faces down to allow for pure growth of reorganizedlayer 18 without any accumulations due to gravity. The hydrothermalprocess causes seed layer 16 to reorganize with the growth ofreorganized layer 18. Reorganized layer 18 may have the same thicknessas seed layer 16. During the process, the morphology changes fromamorphous looking spheres to faceted particles. The ablated surface ofthe first electrode layer 12 composed largely of molecular sievefragments reorganize and seed the gel mixture 52 in order to create theporous sensor layer. The hydrothermal treatment of the ablated surfaceleads to enhanced surface crystallinity of the porous sensing layer.

The fabrication of a capacitance type gaseous sensing device 10 will nowbe discussed with reference to a specific embodiment. A titanium nitridefirst electrode layer 12 is formed on semiconductor substrate 14.Semiconductor substrate 14 and titanium nitride first electrode layer 12are placed on platform 42 of the pulse laser ablation process. Target 24consisting of aluminum phosphate AlPO₄ -5 was prepared by pressingapproximately one gram of the molecular sieve material into a one inchdie. Target 24 is placed on mounting block 26. Excimer laser 30 usingkrypton fluoride radiation generates a 248 nanometer wavelength laserbeam 22 with a power density of 10⁶ W/CM². Laser pulse energies from 50mJ to 400 mJ with pulse repetition rates from 1 to 80 pulses per secondmay be employed. Laser beam 22 is reduced to a diameter of 1.7centimeters by beam reducer 32 before being reflected off of rasteringmirror 28. Rastering mirror 28 is used to position laser beam 22 alongany portion of target 24. Laser beam 22 is further reduced by a 10 inchfocal length convex lens 34 to a 1 millimeter size spot on target 24.Laser beam 22 enters vacuum chamber 36 through quartz window 38 andcontacts target 24 mounted at an angle of 35° relative to laser beam 22.Lamp 44 is turned on to heat semiconductor substrate layer 14 andtitanium nitride first electrode layer 12 to temperatures in the rangeof 150° C. to 350° C. Vacuum chamber 36 is pressurized through gasmanifold and pressure gauge 48 to pressures in the range of 0.15 to 0.45Torr using either oxygen, template such as tripropylamine ordicyclohexylamine, and oxygen, water and oxygen, or water vapor.Ablation of target 24 aluminum phosphate molecular sieve materialgenerated a visible plume of material that varies in size, shape, andcolor depending upon the experimental conditions. The molecular sievematerial of plume 40 deposits onto titanium nitride first electrodelayer 12 as molecular sieve fragments.

In order to enhance the crystallinity of seed layer 16, capacitance typegaseous sensing device 10 is subjected to a post hydrothermal treatment.Capacitance type gaseous sensing device 10 is placed in a teflon linedchamber 50 containing an aluminum phosphate AlPO₄ gel mixture designedto prepare aluminum phosphate AlPO₄ -5. Chamber 50 is heated to atemperature of 150° C. for a process time of 1 to 24 hours. Chamber 50is also pressurized to approximately 30 Psi. During hydrothermaltreatment, the ablated molecular sieve film, seed layer 16, reorganizesunder the hydrothermal conditions with the aluminum phosphate gelmixture 52. Seed layer 16 allows for the deposition of partiallycrystalline aluminum phosphate molecular sieve material of reorganizedlayer 18 onto titanium nitride first electrode layer 12. Selective vapordeposition of gold/palladium onto portions of reorganized layer 18create second electrode layer 20.

FIG. 4 shows a plot of capacitance per square micron for a 70.3nanometer thick aluminum phosphate AlPO₄ -5 molecular sieve measuredfrom between -0.5 to +0.5 volts and an oscillator frequency of 100 kHzexposed to different gas ambients. As seen in the plot, the nitrogenresponse is 100 times greater than the instrument noise level of 10⁻¹⁶farads. Upon exposure to atmospheres of carbon dioxide or toluene, thecapacitance increases nearly 1000 times. The response to carbon monoxideis almost 10 times greater than that of carbon dioxide. This isconsistent with a highly polar carbon monoxide molecule stronglyabsorbing into the molecular sieve material. The plot shows thatcapacitance type gaseous sensing device 10 can discriminate betweendifferent gaseous elements. It is possible to reverse the changes incapacitance of capacitance type gaseous sensing device 10 for laterreuse through temperature or pressure adjustments.

Uniform thin films of low density molecular sieves can be produced onsemiconductor compatible materials, using semiconductor compatibleprocesses. Such a technique allows for the capability to buildintegrated circuits within semiconductor substrate layer 14 for use incontrolling and evaluating capacitance type gaseous sensing device 10.An integrated circuit may be fabricated within semiconductor substratelayer 14 to produce a detection signal when a gaseous element isabsorbed into reorganized layer 18. When the integrated circuit processis complete, the molecular sieve material can be deposited on firstelectrode layer 12. The molecular sieve material can be deposited at thewafer level in order to reduce processing costs. Once the molecularsieve material is deposited, second contact layer 20 is deposited andbonded to the integrated circuit. The entire molecular sieve materialdeposition could be performed as a back end process so that themolecular sieve material would not enter a wafer fabrication. Thisprocess flow provides a completely monolithic silicon based chemicalsensor.

In summary, a capacitance type gaseous sensing device is fabricated byforming a uniform reorganized layer between two electrode layers. Theuniform reorganized layer is formed by forming a seed layer on a firstelectrode layer through pulse laser ablation of molecular sievematerial. Hydrothermal treatment is performed on the device in order togrow a reorganized layer onto the first electrode layer throughinteraction with the seed layer. A second electrode layer is formed ontothe reorganized layer in order to establish the capacitance type gaseoussensing device. In operation, the reorganized layer absorbs gaseouselements which change the dielectric constant of the device. A change indielectric constant causes a change in capacitance in the reorganizedlayer which is measured across the first and second electrode layers.

Thus, it is apparent that there has been provided, in accordance withthe present invention, a method of forming a capacitance type gaseoussensing device and apparatus thereof that satisfy the advantages setforth above. Although the preferred embodiment has been described indetail, it should be understood that various changes, substitutions, andalterations can be made herein. For example, though specific materialsand process parameters have been described, other materials and processparameters may be used and varied in order to achieve reliable sensingproperties. Other examples are readily ascertainable by one skilled inthe art and could be made without departing from the spirit and scope ofthe present invention as defined by the following claims.

What is claimed is:
 1. A capacitance type sensor device, comprising:afirst electrode layer located on a semiconductor substrate layer; areorganized layer uniformly located on said first electrode layer, saidreorganized layer formed of a low density molecular sieve material, saidreorganized layer having a crystalline surface; a second electrode layeron selected portions of said reorganized layer, said reorganized layeroperable to absorb gases into said low density molecular sieve material,said reorganized layer having a dielectric constant that changes inresponse to said gases absorbed into said low density molecular sievematerial, said change in dielectric constant being measurable as achange in capacitance across said first and second electrode layers. 2.The capacitance type sensor device of claim 1, wherein said first andsecond electrode layers include an oxidation resistant inert metal. 3.The capacitance sensor device of claim 1, wherein said low densitymolecular sieve material is selected to absorb carbon monoxide gases. 4.The capacitance type sensor device of claim 3, wherein said low densitymolecular sieve material includes aluminum phosphate.
 5. The capacitancesensor device of claim 2, further comprising:a semiconductor substratelayer, said first electrode layer being formed on said semiconductorsubstrate layer, said semiconductor substrate layer having an electriccircuit operable to monitor said change in capacitance across said firstand second electrodes.